POLLEN DISPERSAL AND INTERSPECIFIC GENE FLOW IN LOUISIANA IRISES

An analysis of chloroplast DNA (cpDNA) variation was carried out for 106 individual plants from three natural populations of Louisiana irises. Two of the samples (59 individuals) represented I. brevicaulis populations. The third sample was from a population defined by allozyme markers as an area of contact between I. fulva, I. hexagona and I. brevicaulis. The cpDNA acts as a seed-specific genetic marker because it is inherited from the maternal parent. cpDNA markers were thus used to discriminate between (i) introgressive hybridization due to seed movement followed by pollen transfer and, (ii) introgression resulting from direct transfer of pollen between allopatric populations of the hybridizing taxa. Furthermore, the concurrent analysis of biparental and maternal markers for the same individuals allowed a test for any directionality in the introgression. A comparison of cpDNA results with data from previous nuclear analyses led to the conclusion that pollen flow occurred from allopatric populations of I. hexagona into an area of sympatry involving I. fulva and I. brevicaulis. In addition, the genotypes detected in the hybrid population indicate that I. fulva and I. brevicaulis have acted as both pollen and seed parents to produce introgressant individuals. The results of the present study and those of previous nuclear and cpDNA analyses suggest that pollen dispersal is the most important avenue for gene flow between these Iris species.